Signal path description leading into XFMEA

submission_thesis/CH2_FMEA/copy.tex

@@ -536,11 +536,71 @@ approach in looking for system failures.
 
 In this section we examine some fundamental concepts and underlying philosophies of FMEA.
 
-\paragraph{The unacceptability of a single component failure causing a catastrophe.}
+
+\paragraph{The signal path.}
+
+Most electronic systems are used to process a signal: with signal processing
+there is usually a clear afferent to transform to efferent path.
+%
+That is, there is an input, some processing and an output.
+%
+Some circuits have feedback loops or even circular signal paths, but it
+is normal for a signal path to exist.
+%
+%can be identified.
+%
+An FMEA investigation will often take the component {\fm} and examine its effect along this path,
+in the direction of the signal,
+echoing diagnostic/fault~finding methods~\cite{garrett, maikowski}. % loebowski}.
+%
+The rationale and work-culture of those tasked to 
+perform FMEA are generally personnel who have performed fault finding.
+%
+When fault finding we generally follow the signal path, checking for correct behaviour
+along it: when we find something out of place we zoom in and measure 
+the circuit behaviour until we find a faulty component or module.
+%
+With this style of fault finding, because it is based on experiment, 
+we can hop from module to module eliminating working modules, until we find the 
+failure.
+
+
+FMEA is a theoretical discipline. It  would be very unusual to build a circuit and then simulate
+component failure modes. This would be very time consuming as it would involve building a circuit for each component {\fm} in the system.
+%
+We cannot, as with fault finding, verify modules along the signal path for correct behaviour
+and eliminate them from the investigation.
+%
+With FMEA we therefore need to be more thorough.
+%
+The question is by how much.
+Too much and the task becomes impossible due to time/labour constraints.
+Too little and the analysis could become meaningless because it misses
+potential system failures.
+%
+For a more complete analysis we should perhaps, examine each component {\fm} along the complete signal path, 
+forwards and backwards from the placement 
+of the component exhibiting the {\fm} under investigation.
+%
+Also, whether following the effects through the signal path {\em only} is acceptable, and instead
+looking at its effect on all other components in the system is necessary,
+is a matter for debate, and is examined in section~\ref{sec:xfmea}.
+
+
+\paragraph{Single component failure mode to system failure relation.}
+
+
 % NEED SOME NICE HISTORICAL REFS HERE
 FMEA, due to its inductive bottom-up approach, is good
 at mapping potential single component failures to system level faults/events.
-Used in the design phase of a project, FMEA is a useful tool
+%
+The concept of the unacceptability of a single component failure causing a system failure, % catastrophe,
+is an important and easily understood measurement of safety. 
+%
+It is easy to calculate
+because we can usually find Mean Time to Failure (MTTF) statistics for commonly used components.
+%
+Also, used in the design phase of a project, FMEA is a useful tool
 for discovering potential  failure scenarios~\cite{1778436820050601}.
 %
 % Subject Object Wiki answers : Best Answer
@@ -655,7 +715,7 @@ methodologies.
 % cite for forward and backward search related to safety critical software
  %{sfmeaforwardbackward}
 \subsection{FMEA and the  State Explosion Problem}
-
+\label{sec:xfmea}
 \paragraph{Exhaustive Single Failure FMEA.}
 
 FMEA for a safety critical certification~\cite{en298,en61508} will have to be applied